This type of device comprises a large number of photosensitive points called pixels generally organized as a matrix or strip array. In a detector, a pixel represents the elementary sensitive element of the detector. Each pixel converts the electromagnetic radiation to which it is subjected into an electrical signal. The electrical signals arising from the various pixels are collected during a phase of reading the matrix and then digitized so as to be able to be processed and stored to form an image. The pixels are formed of a photosensitive zone delivering a current of electric charges as a function of the flux of photons which it receives, and of an electronic circuit for processing this current. The photosensitive zone generally comprises a photosensitive element, or photodetector, which may for example be a photodiode, a photoresistor or a phototransistor. Photosensitive matrices of large dimensions which may possess several million pixels are found.
The radiation detector can be used for the imaging of ionizing radiations, and notably X rays or γ rays, in the medical sector or that of nondestructive testing in the industrial sector, for the detection of radiological images. The photosensitive elements make it possible to detect a visible or near-visible electromagnetic radiation. These elements are hardly, if at all, sensitive to the radiation incident on the detector. Use is then frequently made of a radiation converter called a scintillator which converts the incident radiation, for example X-ray radiation, into a radiation in a band of wavelengths to which the photosensitive elements present in the pixels are sensitive. An alternative consists in producing the photosensitive element from another material carrying out the direct conversion of the X-ray radiation into electric charges. This is the case for example for matrices in which a first pixellated substrate made of Cadmium Telluride (CdTe) is connected pixel by pixel to a CMOS reading circuit which therefore no longer possesses the detection function.
It is known to produce an electronic processing circuit by means of a voltage follower making it possible to read the current of charges accumulated in the photosensitive element. A current source ensures the power supply for the pixel during its reading. An exemplary imaging device thus produced is represented in FIG. 1.
This figure schematically presents a matrix of two rows and two columns to simplify understanding. Four pixels are formed, each at the intersection of a row and column. Of course the real matrices are generally much larger.
Each pixel comprises a photosensitive zone represented here by a photodiode D and an electronic processing circuit formed of three transistors T1, T2 and T3. In the figure, the labels of the photodiode D and of the three transistors are followed by two coordinates (i,j) that can take the rank of the row for i and the rank of the column for j.
The pixels of one and the same column or more generally of one and the same row share a transistor T4 and a reading circuit S situated at the column end. The transistor T4 and the reading circuit S are linked to the pixels of the column by means of a conductor Col. The pixels of one and the same row are joined to four conductors conveying signals Phi_line, Vdd, V_ran and Phi_ran making it possible to control each of the rows of pixels.
The transistor T1 makes it possible to reinitialize the voltage of the cathode of the photodiode, to the voltage V_ran, during a phase of reinitializing the matrix during which the control signal Phi_ran is active.
After reinitialization, the illumination received by the photodiode D causes the potential of its cathode to decrease during an image capture phase.
This image capture phase is followed by a reading phase during which the potential of the photodiode D is read. Accordingly, the transistor T3 is turned on, the latter therefore having a role as switch, by virtue of the command Phi_line applied to its gate.
The transistor T2 operates as follower, and the transistor T4 operates as current source. The transistors T2 and T4 then form a voltage-follower stage which copies the voltage present on the cathode of the photodiode D, and reproduces it, to within a shift, on the input of the reading circuit S at the column end. To carry out its copyover, the transistor T2 requires a bias current flowing in its drain and its source. This current is imposed by a current generator formed by a transistor T4 common to several pixels. In the example represented, the transistor T4 is common to a column of pixels.
The voltage Vs present at the input of the reading circuit S can be expressed:Vs=Vp−VT−K  (1)
Where Vp is the voltage of the cathode of the photodiode, VT is the threshold voltage of the transistor T2, and K is a constant related inter alia to the value of the current delivered by the transistor T4.
The voltages V_ran and Vdd are often identical.
The phase Phi_line of a row n of a matrix is often the same as the phase Phi_ran of the preceding row n−1. In this case the integration period for the signal upstream of the follower lasts, for row n−1, from the end of the addressing of row n, until the addressing of row n−1 at the following image. The reinitialization and reading phases are therefore different for each row. One speaks of circular addressing of the rows, well known in the literature by the name “rolling shutter”.
The addressing circuits (generally shift registers) generating the control signals Phi_line and Phi_ran are not represented in the figure and are disposed at the row end.
The various outputs of the reading circuits S of the various columns are thereafter multiplexed in a register, not represented in the figure, so as to obtain a video signal of a row.
It is also possible to use just a single current-source transistor T4, for the whole of the matrix, on condition it is switched successively onto the various columns, in tandem with the reading of these same columns.
In practice, each column Col exhibits a lineal resistance represented in the form of a resistance R_pix for each pixel. Relation (1) is correct only at the level of the output node of the pixel, that is to say at the level of the source of the transistor T2. But when this voltage is situated at the end of the columns, on the input of the reading circuit S, it is marred by an ohmic shift related to the resistance of the transistor T3 and to the number n of pixels separating the pixel selected from the reading circuit S along the column Col. The number n corresponds to the rank of the pixel read in the matrix. More precisely, the voltage Vs is expressed:Vs=Vp−VT−K−I×(R(T3)+n×R_pix)  (2)
The ohmic shift is troublesome since it depends on the rank of the pixels read and it therefore introduces a variable skew into the reading of the voltages Vp of the photodiodes.
A solution consists in decreasing the lineal resistance value R_pix of the columns Col by increasing the width of tracks forming the column Col conductors. This solution nonetheless presents several drawbacks.
The increase in width of the tracks uses some surface area of the substrate on which the matrix is produced, and therefore reduces the useful surface area in each pixel for photodetection.
The increase in width of the tracks also increases their electrical capacitance. Now, the voltage of the columns changes upon the addressing of each new row, since it represents at each row the illumination of the corresponding pixels. Increasing the electrical capacitance of the column therefore makes it necessary to feed (or to extract) further charges upon each change of row. This increase in the current increases the ohmic drop, thereby reducing the effect initially sought. Ultimately, this is manifested by an increase in consumption, or by a limitation of the reading speed.
The structure of such a pixel with three transistors therefore finds a limit in respect of matrices of large dimensions in which the column capacitances and the column resistances are significant. These matrices cannot be read rapidly.
Another solution consists in replacing the current sources, common to a column, with current sources disposed in each pixel by means of an additional transistor such as for example described in patent application No. WO2009/043887.
The output column Col conductor then no longer has the role of conveying the bias current for the follower stage. It is used only as a device for observing the output voltage of the pixel. It works at zero current, outside of the current transients necessary at each row transition to establish the new voltage value. At the end of these transients, the current on the column conductors being zero, the voltage is the same over the whole of the length of these columns, and in particular the voltage at the column end does indeed represent, with no shift, the voltage of the pixel. The lineal resistance R_pix no longer therefore causes any voltage drop along the column conductor.
This solution nonetheless has the defect, by introducing a current-source transistor into each pixel, of requiring the corresponding control voltages on the gate and the source of this transistor.
Distribution of the gate voltage over a matrix is done without any ohmic drop, since there is no corresponding consumption. But the distribution of the source voltage undergoes ohmic drops since it must generate the bias current for the follower transistor T2.
The source voltage of the additional transistor varies from one pixel to the next as a function of the lineal resistance of the conductor conveying this voltage. The potential difference between gate and source defines the value of the current delivered by these transistors to bias each follower transistor T2. These variations in bias current from one pixel to another therefore create, as in the pixel with three transistors, dispersions on the output signals.
It is possible to alleviate this drawback by keeping the potential difference between source and drain of the additional transistor constant for all the pixels. For this purpose, a voltage drop equal to that which exists on the sources of the current-source transistors is created on their gates, by connecting them to a lateral resistive bar which reproduces the same voltage drop as that which exists on the sources of the additional transistors as described in patent application FR 2 921 788. The resistive bar is linked to all the gates of the additional transistors of a column and is supplied between two voltages, high and low, at the ends of the bar.
This solution operates well, but is fairly complex to put in place and to pilot, since it requires two voltages, high and low (Vg1, Vg2) for biasing the lateral resistive bar.
Another solution for circumventing the problem of ohmic drop has for example been proposed in patent application EP 1 416 722 A1. This solution consists in displacing the current source onto the opposite edge of the matrix, with respect to the edge on which the reading circuits S are situated.
The bias current for the pixel goes from the selected pixel up to the current source, and therefore creates an ohmic drop in the part of the column conductor above the selected pixel. But the lower part, connected to the reading circuit S, is not traversed by any current, and does not therefore undergo any ohmic drop. The lower end of the pixel is therefore properly representative of the voltage of the pixel.
However, this solution exhibits several limitations. It is necessary to have the space required to produce the source at the top of the column. And in the case where several matrices have to be assembled or abutted, for example in fours, so as ultimately to obtain a sensitive area four times as large, then the top of the column is situated at the limit of the desired smallest possible abutment zone. It is therefore not desirable to place in this abutment zone components not belonging to the actual pixels. This would distance the sensitive parts of the elementary matrices, thus creating a dead line in the final image at the level of the abutment.
Moreover, the voltage drop between the addressed pixel and the current source must not be too large, so that the current-source transistor remains in saturated mode, and properly fulfills its current-source function. And as described previously, if the width of the column is increased in order to reduce this voltage drop, this will increase consumption.